Abstract
The pretreatment of soils with pH5 sodium acetate (NaOAc) buffer is widely used to remove carbonates from soils prior to dispersion and separation of clays for mineralogical analysis. The effect of this pretreatment on other minerals such as palygorskite and zeolite (analcime) was evaluated. Soil samples from Arizona and Egypt as well as reference samples of palygorskite and analcime spiked with CaCO
3
were used. The pretreatment of the soils and reference samples had no observable change in the mineralogy as indicated by X-ray diffraction and SEM analysis. The study further suggests that the somewhat tedious and time consuming step can be avoided for the mineralogical analysis of many arid land soils.
The common procedure for clay separation of calcareous soils for mineralogical analysis and study by X-ray diffraction is to remove the carbonates by a pH5 sodium acetate pretreatment
(1,2)
. For soils low in carbonates, the pretreatment may be feasible, but for highly calcareous soils the procedure is time consuming and tedious with the inherent risk of altering some minerals.
Among the minerals found in certain calcareous soils that are usually considered to be susceptible to acid deconposition and alteration are sepiolites, palygorskites and zeolites
(3)
. Palygorskites and sepiolites essentially occur in alkaline conditions (pH 8 to 10) in the presence of salts and high activity of Si and Mg, but little if any Al, a condition common to many desert soils. Such minerals have been reported in soils in the southwestern United States, such as calcareous C horizons in New Mexico
(4)
, in "caliche"; horizons as well as in the sola of many calcareous soils on the southern High Plains in west Texas and eastern New Mexico
(5)
,many parts of the middle-east
(6,10)
, and curtain soils of Australia
(11)
. Analcime (a zeolite) has been identified in a number of soils in the San Joaquin valley of California
(12)
in strongly saline soils developed from granitic alluvium with a pH above 9 and containing N
2
CO
3
.
The purposes of this study were to determine: i) the effect of pH5 NaOAc pretreatment for carbonate removal on soil and specimen clays containing palygorskite, analcims and other clay minerals, and ii) to evaluate the usually recommended procedure for treating calcareous soils for the purpose of clay separation and X-ray diffraction analysis